1. Field of the Invention
This invention concerns both a system and a method for removing the spent fuel rods from a fuel rod assembly, and consolidating these rods into a maximum density array within a storage cannister.
2. Description of the Prior Art
Tooling systems for removing the fuel rods from a nuclear fuel rod assembly are known in the prior art. Such tools are often used to remove the spent fuel rods from a fuel rod assembly so that they may be consolidated into a storage cannister, and ultimately placed in the spent fuel pool of the nuclear power plant facility. To this end, such tooling systems typically include a rod gripping mechanism for selectively gripping and ungripping one or more fuel rods in a fuel rod assembly after the top nozzle of the assembly has been removed. Such rod gripping mechanisms are connected to a crane-like mechanism and operate by lifting the rods out of the assembly by applying a tensile or pulling force, and lowering them into a storage cannister. An example of such a fuel rod gripping mechanism is disclosed and claimed in U.S. Pat. No. 4,651,400 filed Dec. 21, 1983 by Edward Shields and assigned to the Westinghouse Electric Corporation. While such fuel rod removal systems are most commonly used in the context of spent fuel consolidation processes, they may also be used to remove the unspent fuel rods from a damaged fuel rod assembly incident to a reassembly-type repair.
Unfortunately, the performance of tooling systems that remove the fuel rods from their respective assemblies by gripping and pulling them out is not without shortcomings. However, before these shortcomings may be fully appreciated, some brief background and to the structure, operation and environment of such fuel rod assemblies is necessary.
Nuclear fuel rod assemblies generally comprise between 200 and 290 fuel rods mounted in a square array within a support skeleton. The support skeleton in turn is formed from top and bottom nozzles which are interconnected to one another by sixeen to twenty-four uniformly arrayed thimble tubes. The top and bottom nozzles are eight to nine inches square, and the thimble tubes are about thirteen feet long, so that the overall shape of the fuel assembly is that of an elongated, rectangular prism (see FIG. 1). The fuel rods themselves are about twelve feet long. In order to equidistantly space the long and relatively flimsy fuel rods within the support skeleton, the skeleton includes between seven and nine grids, each of which has a square array of open cells for receiving and spacing the fuel rods. The grids are usually formed from flat plates of interlocking sheet metal in an "eggcrate" configuration which lends compressive strength to the grids with a minimum of weight. When such fuel assemblies are first placed into operation, they are lowered into the reactor core by a crane, and the neutron-absorbing control rods which are interspersed between the fuel rods are slidably removed. Pressurized water is then conducted through the bottom nozzles of the fuel assemblies in order to uniformly absorb the heat generated by the nuclear reaction that takes place between the fuel rods. Over a period time, the nuclear fuel within the fuel rods becomes exhausted, thereby necessitating the removal of the spent fuel assembly from the nuclear core, and the disposal of its fuel rods in the spent fuel pool of the nuclear power plant facility. However, in order to make optimum use of the limited amount of storage space available in the spent fuel pool, the spent fuel assembly is first taken to a cask loading area of the pool for consolidation. The cask loading area of the pool is approximately forty feet deep, and filled with water. The water shields workers who typically stand on a deck located above the pool from radiation. Once the spent fuel assembly is loaded into the cask loading shaft, the workers remove at least the top nozzle of the assembly, and then utilize a gripping device (such as that disclosed in the previously mentioned U.S. Pat. No. 4,651,400) to grip and withdraw all of the spent fuel rods from the fuel assembly in order to load these rods into a storage canister. Because of the relatively low thermal output of spent fuel rods, empirical studies have shown that they may be packed in parallel in contact with one another in a " triangular" array (shown in cross-section in FIG. 4E), which is the densest possible arrangement for a plurality of rod-like objects. In such an arrangement, the axes of rotation of the rods (when viewed in plan cross-section) appear to define the corners of equilateral triangles; hence the name "triangular" array. The triangular arrangement of spent fuel rods advantageously reduces the volume that these rods occupied within the fuel assembly by a factor of 50%, which in turns allows the spent fuel pool to hold over twice as many spent fuel rods. While prior art tooling systems are known which are capable of effectively removing and consolidating the fuel rods from spent fuel assemblies into storage cannisters, the operation of such tooling systems is not without its costs and risks. For example, the gripping, raising and lowering of spent fuel rods from such assemblies is a slow and tedious process. While the water in the cask loading area of the pool does afford an effective shield for the majority of radiation emanating from the fuel being consolidated, the workers on the deck still receive some amount of potentially hazardous radiation largely due to the length of time necessary to complete the consolidation operation. Additionally, the tensile forces applied to the spent fuel rods when they are being forcefully pulled out of the grids of the fuel rods assembly can cause the relatively brittle outer tube of Zircaloy.RTM. cladding to break, thereby contaminating the water in the spent fuel pool with pellets of radioactive uranium oxide. Finally, the complete withdrawal of the spent fuel rods from the fuel assembly requires these rods to be hoisted upwardly within the spent fuel cask at least fifteen feet. If any of the gripper mechanisms holding the rods should slip at this juncture, one or more of the rods could fall to the bottom of the pool, and break.
Clearly, a system for removing fuel rods from a fuel rod assembly is needed that is both faster and safer than prior art systems. Ideally, such a fuel rod removal system should be adaptable to any size or type of fuel rod assembly now in commercial use. Finally, it would be desirable if such a system were mechanically simple yet extremely reliable despite the differences in sizes of the individual fuel rods so as to minimize the occurrence of broken fuel rods during the fuel consolidation operation.